Interpretive Summary: Soil erosion in Southeastern Spain is a serious problem, due to the establishment of terraced orchards on constructed steep slopes. Slopes are steep, rainfall events can be severe, and soils are highly disturbed and erodible. This research examined the effects of the use of three different kinds of plant covers (thyme, lavender, and native spontaneous vegetation) grown on the terrace taluses (steep slope below flat terrace area). Bordered erosion plots were constructed in the taluses, and runoff, soil loss, and nutrient losses were measured following natural rainfall events. All three types of vegetation had measured runoff and soil loss that were well below those observed from bare soil plots at the site in another experiment. The lavender and native vegetation had the lowest observed losses of soil nutrients, compared to those from the thyme treatment. The thyme and lavender also produced yields of aromatic oils, a source of cash income for farmers in the region. Thus, in addition to the soil and water conservation benefits, these plants also can provide economic benefits. The lavender treatment resulted in lower runoff, soil loss, and nutrient losses compared to the thyme, as well as greater oil production, so it may be a better alternative plant for use in erosion control here. This work impacts farmers, conservation agency personnel, and others involved in efforts to reduce soil loss from man-made terraces on steep slopes. Use of vegetation on the taluses of terraces can greatly reduce runoff, soil loss and nutrient loss from the slopes, protecting the on-site soil resource and off-site water quality.

Technical Abstract:
Southeastern Spain, particularly the coast of Granada and Málaga, is an important region for subtropical cultivation. Orchards have been established there on the slopes of the mountainous areas in constructed terraces. The climate is characterized by heavy periodic rainfall that is variable in space and time, and the common agricultural practice is to leave bare soil on the terrace taluses, both of which are main factors affecting soil erosion, runoff, and subsequent nutrient losses. Over a two-year period, three plant covers: (1) Thymus mastichina (thyme), (2) Lavandula dentata (lavender), and (3) native spontaneous vegetation were tested to determine their effectiveness in reducing erosion and potential risk of pollution by agricultural runoff. The 16 m2 erosion plots (4 m x 4 m) were laid out in the taluses of the terraces. The lowest runoff and soil erosion rates, ranging from 0.01 to 3.1 mm and 0.1 to 25.0 g m-2, respectively, over the entire period, were measured from the native spontaneous vegetation plots. Average annual runoff rates from the thyme, lavender, and native vegetation treatments were 16.5, 15.1, and 7.5 mm yr-1, respectively, and annual erosion rates were 2.3, 1.9, and 1.0 Mg ha-1 yr-1. All of the vegetated treatment rates were considerably lower than those measured from taluses with bare soil for runoff and soil erosion in a similar earlier experiment (100 mm yr-1 and 9.1 Mg ha-1 yr-1, respectively). The overall total losses in runoff from the thyme, lavender, and native vegetation treatments were 36.7, 22.4, and 31.4 mg m-2 yr-1 of inorganic nitrogen (N); 24.1, 4.9, and 1.3 mg m-2 yr-1 of phosphorus (P); and 70.8, 60.6, and 47.5 mg m-2 yr-1 of potassium (K), respectively. Thus the lavender and native vegetation reduced total average N loss by 1.6 and 1.2 times, respectively, compared to the thyme treatment. The native vegetation reduced phosphorus and potassium losses compared to thyme and lavender by 19 and 3.8 times, and 1.5 and 1.3 times, respectively. The potential essential oil yield from the aromatic plant covers for the study period was 47.3 and 122.5 L ha-1 for thyme and lavender, respectively. Thus, cultivation of thyme and lavender in the taluses of the orchard terraces could provide economical as well as environmental benefits for local farmers. This work highlights the benefits of plant covers in controlling on-site erosion effects (soil loss and eventual collapse of the terraces) and off-site effects of agricultural runoff (non-point source pollution).